Tech crew blowing speakers too frequently...

[TimoteusR]

From the discussion on LAB and the posts by Dave himself I came to the conclusion that there are two kinds of amps. First design uses one power supply per channel. This means more headroom (channels don't compete for power) and also bigger costs. The second design uses one power supply for all amp channels. By default both channels are wired the same way. This means if you feed both with the same signal both channels will pull current from + at the same time. If one of the channels polarity is reversed it will do exactly the opposite of the other channel, thus pulling + while the other pulls -. The key here is the common power supply for both channels. With different ps for each channel it doesn't make a difference While I haven't studied the different amp classes I'm pretty sure feeding out of polarity signal to a amp will not damage it. Sure 10% isn't a big change. But when your underpowering even that little bit helps.

[Nephilim]

Ah, misunderstood there. Yes. With AB and regular switchmode amps you'll get a small boost of headroom by doing the polarity jiggery-pokery mentioned. Class-I doesn't need it. Then again, Class-I is freaking expensive.

[mr_sound]

Originally Posted by tenor_singer

2. I think that they are popping because some fool always trips on the plug and unplugs the system from the wall sending a loud "thwack" through the speakers. Could this be causing it? If so that was this fall's reason.

rule #1 i tell everyone with live sound.....ALWAYS ALWAYS ALWAYS turn the power amps on LAST, and off FIRST. all those little power up thuds you get in the signal chain sent through a 10,000 watt rig will surely blow things up...and if they don't you can consider yourself lucky. a little tip for power problems...tape down cables and tie the cable down so no one can accidentally trip over it. too much time has been spent fixing something as simple as an unplugged cable. been there, done that.

3. This spring, my TD turned on a body mic before checking it's volume on the board, which was pegged (some jack-off was messing around with the slides when she wasn't looking). The feedback that went through the system was amazing (unfortunatelly she didn't have the destroyer programmed yet). Would this cause the speakers to pop?

you should really have limiters on all your mic channels. if you can't afford that, a good brick wall limiter on the main output will due. i'm paranoid, so i use channel limiters on bands with screaming singers, always have the main limited, have my clip limiters on the power amps turned on, and i still keep an eye on those clip lights. which brings me to my next suggestion....can you see the power amps from the mix location? if so, you can make sure your amps aren't clipping. i don't know if your amp has a clip indicator, but most do. it'll probably be red. if it lights up a little bit, it's ok. if it lights up a lot....turn down the mix. even if you don't have proper gain structure, and don't keep an eye on your levels at the board...watching the clip lights on the amp will save your speakers. of course, if a channel is clipping heavily the mix will sound like shit...but you won't blow anything up.

BTW, feedback destroyers don't work all that well. their performance varies depending on what model you have, and even then they aren't failsafe. most pros don't even use them. i do on monitor sends since i can't afford a graphic for every monitor mix, and it does help a bit for the singer who likes to roam, but if the case is a mic that is turned up way too high, the feedback destroyer will not stop this. a limiter on the channel though will stop it from getting worse.

i recommend you get at least 1 limiter. behringer would work pretty well for your case. get the 4 channel compressor/limiter. you can use 2 channels for vocal compression, and use the other 2 for your stereo out. you could even compress your stereo out mix a little bit with this thing....if you wanted to...i don't know if there's much use for that in theater.

[Eboy87]

I have a question that sort of relates to this. A band that I run sound for blew out a tweeter (it might have been an HF horn) on a JBL. I'm not sure what the model was. I have an Alesis 3630 compressor/limiter. If I wire it in series with the speakers, will it prevent this from happening again? or would the power ruin the compressor?

[DMXtools]

Originally Posted by Eboy87

I have a question that sort of relates to this. A band that I run sound for blew out a tweeter (it might have been an HF horn) on a JBL. I'm not sure what the model was. I have an Alesis 3630 compressor/limiter. If I wire it in series with the speakers, will it prevent this from happening again? or would the power ruin the compressor?

Having a compressor in the system can certainly help, but you need it between the board and the power amps. Placing it between the amps and speakers could very easily damage the poor 3630.

An important thing is the size of the amplifier - how many watts. If your amp is too big for the speakers, there's a good chance it'll be able to smoke the woofers. If your amp is too small for the speakers, it'll be easy to overdrive it, causing distortion - which is hell on tweeters.

Overdriving an amp causes clipping - a form of harmonic distortion that packs a lot of high-frequency energy. A 200-watt amp driving a speaker system rated for 300 watts probably won't hurt the woofer - chances are it's rated for 300 watts all by itself. But the tweeter may only be rated for 50 watts. If you overdrive the amplifier, it's packing most of it's power into the high frequencies, easily blowing a tweeter.

Assuming you don't drive your mixer itself into clipping (which has the same effect), a compressor between the mixer and power amp can be set to keep the level below what would cause the amp to clip.

John

[halojen]

i think the point that was made earlier is that clipping doesnt directly cause DC to go to your speaker.
i think the story is that clipping causes your amp to break >> then the broken amp outputs DC because its screwed.

[DMXtools]

Originally Posted by halojen

i think the point that was made earlier is that clipping doesnt directly cause DC to go to your speaker.
i think the story is that clipping causes your amp to break >> then the broken amp outputs DC because its screwed.

It takes a lot of clipping to damage a good amp but yes, this is a scenario that can blow speakers, especially woofers or subs. Unless your system is biamped or triamped, and sometimes even then, tweeters generally have a capacitor in series with them that will block the DC. It's the high power at high frequencies, caused by the clipping before the amp blows, that will wipe out most tweeters.

Many Peavey speakers (and some others, I assume - I'm just more familiar with Peavey) have a protective circuit for the high-frequency driver (tweeter). They simply place a small incandescent lamp in series with the tweeter.

It works because of a characteristic of tungsten called "positive temperature coefficient" - that is, as the temperature rises, the resistance rises. When the filament is cold (at normal volumes), its resistance is very low. At excessive volumes it heats up and lights up, the resistance goes up rapidly and more of the power from the amp is dissipated in the filament than in the speaker. In an extreme case, the lamp may blow, acting as a fuse. Usually it doesn't, and as soon as the volume goes down/feedback stops, the lamp cools down rapidly and normal operation resumes.

For my 22XT horns, they use a #1156 lamp, available for about a dollar at any auto-parts store - it's normally used for backup lights, sometimes for turn signals. For the last five years, all my tweeters have had an 1156 in series. In that time I've blown two lamps, but no tweeters. Considering that a replacement diaphragm for a 22XT is about $40, I think two bucks' worth of lamps is a good investment.

John